Two kinds of vacuum gauges are known: thermionic emission vacuum gauges (also called hot cathode vacuum gauges), and field emission (or cold cathode) vacuum gauges.
In thermionic emission vacuum gauges, the electron source comprises in one or more filaments, for instance of tungsten, brought to incandescence. A typical example of hot cathode vacuum gauge is the Bayard-Alpert vacuum gauge. That kind of vacuum gauge comprises a wire-shaped ion collecting plate, a cylindrical grid surrounding the plate and an incandescent tungsten filament for electron emission, located outside the grid. The electrons emitted by the filament and accelerated by the grid ionise the residual gas, and the ions and/or the ionised positive molecules are collected by the plate, which is kept at lower potential than the electron source and the grid.
In the disclosed design, the electrons pass several times through the grid and, during such “in” and “out” movement, they ionise the residual gas until they hit the grid and are absorbed by it.
A plate, which is designed as a simple wire allows for pressure measurements as low as about 10−9 Pa. Indeed, because of the reduced plate wire surface, the background current is minimised due to photoelectric effect from the plate (electron emission) caused by X-rays produced by electrons hitting the grid.
The example of vacuum gauge is disclosed for instance in U.S. Pat. No. 2,605,431 “Ionisation Vacuum Gauge”. The major drawback of that kind of vacuum gauges is related to the nature of the electron-emitting cathode. Actually, a heated filament is an isotropic electron source, where directionality of the electron beam is an essential parameter for vacuum gauge sensitivity.
The vacuum gauge sensitivity is not constant, since the distribution of the electron emission changes direction as the temperature along the emitting cathode filament changes, this filament typically reaching temperatures up to about 2000° C.
Moreover, the phenomenon of electron emission by thermionic effect entails high power consumption, long response times and a non-negligible pollution of the surrounding environment due to the release of impurities.
It is the main object of the present invention to overcome the above drawbacks, by providing a miniaturised vacuum gauge, which has a great sensitivity and which does not appreciably perturb the pressure measurements.
The above and other objects are achieved by a vacuum gauge as claimed in the appended claims.
Advantageously, the gauge according to the invention exploits the nanotube technology for making the electron-emitting cathode.
According to such a solution, electron emission takes place by field effect, and not by thermionic effect: application to a nanotube film of a strong electric field, whose flow lines are concentrated at the ends of the nanotubes, results in electron emission.
A nanotube cathode is a so-called “cold” electron source, requiring very low power consumption and having high directionality.
According to a preferred embodiment of the invention, due to the use of such a cathode, it is possible to utilize not only cylindrical geometry of the conventional Bayard-Alpert vacuum gauge but to use different geometries, allowing miniaturising the ionisation vacuum gauge.
More particularly, according to some embodiments of the invention, the electrons continue moving in the space between the grid and the plate, without any appreciable electron amount passing again through the grid.
The preferred embodiments of vacuum gauge according to the invention, given by the way of non-limiting examples, is disclosed in greater detail hereinafter, with reference to the accompanying drawings, in which: